1. Field of the Invention
The present invention relates to the deposition of thin films of material on a substrate, and more specifically to the deposition of device quality hydrogenated amorphous silicon (a-Si:H) containing low hydrogen content for use as photovoltaic and other semiconducting devices.
2. Description of the Prior Art
In the manufacture and construction of microelectronic semiconductor devices and photovoltaic solar cells, amorphous silicon is often a feasible alternative to the use of silicon crystals for layers of a device, due to economics, flexibility in manufacture, and higher through-put. However, amorphous silicon tends to react chemically with its environment, causing it to become contaminated, and thereby deteriorating the chemical, electrical, and mechanical properties of the intrinsic or undoped silicon. This reactivity of the amorphous silicon can be passivated by the incorporation of hydrogen into the amorphous silicon layer, which is usually accomplished during the deposition process, and it considerably improves the electrical properties of the individual layers and the device.
Two measures of these electrical properties of hydrogenated amorphous silicon layers are the Urbach tail width and the density of midgap states, both of which should be minimized to achieve device quality semiconductor films. Although exact mechanisms are not known, there has appeared to be a relationship between the amount of hydrogen incorporated and both the Urbach tail width and density of midgap states. At hydrogen concentrations too low, the amorphous silicon film exhibits very poor electrical properties due to the high density of midgap states and is thus not suitable for use in practical devices. At hydrogen concentrations too high, these films show an increased density of microvoids and once again inferior electrical properties.
Incorporation of this hydrogen into the amorphous silicon film is not without its costs, however. Specifically, when this hydrogenated amorphous silicon is used in photovoltaic solar cells, these solar cells over time degrade electrically upon exposure to sunlight. This degradation, which is referred to as the Staebler-Wronksi effect, has been strongly linked to the concentration of hydrogen within the amorphous silicon film. The prevalent model for the Staebler-Wronksi effect suggests that the degradation is due to movement of hydrogen within the film.
In the last decade or so, since the development of the glow discharge (GD) technique as the standard means for producing device quality hydrogenated amorphous silicon films for solar cells and other applications, there has been considerable progress made in increasing the efficiencies of these solar cells. However, most of this progress has been in improved techniques in manufacturing and utilization of these solar cells, such as better uniformity of deposition, better light utilization, and better doping of layers. There has been little or no significant improvement in the quality of the intrinsic or undoped material. There is a general belief in the industry that the present intrinsic material quality of the hydrogenated amorphous silicon film has already been optimized. Yet, as indicated above, there is still room, in fact a need, for additional attention and improvement to this technology to increase efficiencies and usefulness, particularly for solar applications.